Solid organic materials ranging from synthetic polymers to biological tissue are decomposed by intense concentrations of ultraviolet radiation. In contrast to visible and infrared radiation, which emit photons that do not possess sufficient energy to excite bonding electrons between constituent molecules of most organic polymers, ultraviolet radiation emits photons that are absorbed by organic materials in the form of electronic transitions that break apart molecular bonds of organic polymer chains. In fact, at certain power densities of ultraviolet radiation, interaction of ultraviolet laser pulses with an organic substrate results in expulsion of small fragments of the polymers from the substrate at supersonic velocities. This phenomenon is termed "ablative photodecomposition".
The phenomenon of ablative photodecomposition was first disclosed in the patent literature in U.S. Pat. No. 4,414,059 (Blum et al.) in which I am named co-inventor. Previously, it was known that organic solids could be cut (albeit less effectively) by ultraviolet light at low concentrations (i.e., much less than 1 W/cm.sup.2) to produce oxidative photodecomposition. Photons of the ultraviolet light at a wavelength that is absorbed by the organic material break apart molecular bonds of the organic material, and the fragmented polymer chains are oxidized to prevent the chains from recombining. Etching rates by oxidative photodecomposition are quite slow because oxygen does not diffuse into significant depths of the organic material.
It was also known that organic materials could be cut by ultraviolet light at much higher concentrations (i.e., much greater than 1 W/cm.sup.2) to produce photothermal decomposition. Intense concentrations of ultraviolet light at wavelengths that are not absorbed by the organic material react in the same manner as visible and infrared radiation by producing heat that accumulates in the organic substrate causing thermal decomposition. In fact, photothermal decomposition is actually a process whereby the organic material is burned by the accumulation of energy deposited by the ultraviolet light. Although organic materials may be readily cut by photothermal decomposition, thermal damage is also done to the surrounding substrate material. It is also not possible to control depth of cutting into the substrate material with the same precision as processes producing oxidative photodecomposition.
The process of ablative photodecomposition preserves cutting precision of oxidative photodecomposition but increases etching rates by at least thirty times the rates of photodecomposition without detectable thermal damage to the organic substrate. Two main conditions have been identified for initiating ablative photodecomposition in organic materials. First, the wavelength of ultraviolet radiation must be within a range that is efficiently absorbed by the organic material. Second, the radiation source must provide a sufficient number of photons in a short amount of time (i.e., greater than 1 MW/cm.sup.2) so that polymer chains are broken into volatile fragments that evaporate or escape from the substrate.
The organic material is decomposed by electronically exciting the constituent bonds of the polymer chains, thereby breaking the chains into volatile fragments. The fragments are understood to require a larger volume than the unbroken polymer chains of the substrate and "explode" from the substrate carrying away kinetic energy. Although ablative photodecomposition has a critical dependence upon wavelength and power density, neither the presence of oxygen nor the temperature of the organic material is critical to the process.
Excimer lasers are the most common source of ultraviolet radiation at the power densities required for ablative photodecomposition. The pulse width of excimer lasers is generally in the range of 5 to 35 nanoseconds (full width at half maximum), but some experimental work has been done with pulses extending up to 300 nanoseconds. Since pulse widths are constant during most laser applications, power densities are usually measured in terms of "fluence" (i.e., energy per unit area) per pulse. Further, a study entitled "Effect of Optical Pulse Duration on the XeCl Laser Ablation of Polymers and Biological Tissue", published in the Journal of Applied Physics, Jun. 22, 1987, found that threshold values for inducing ablative photodecomposition are fluence dependent within a range of pulses from 7 to 300 nanoseconds. Most organic materials have an ablative photodecomposition threshold fluence of at least 10 mJ/cm.sup.2 over the range of normal pulse widths.
It has been postulated that continuous wave radiation could also be used to initiate ablative photodecomposition. However, pulsed radiation sources must currently be used to provide power densities required for ablative photodecomposition. That is, although the threshold for ablative photodecomposition has been found to be largely independent of pulse widths within the normal range of excimer laser operation, continuous wave lasers (i.e., cw lasers) that are focused on organic materials at lower power densities for extended periods of time have not successfully initiated ablative photodecomposition.
A study of etching organic polymer films on semiconductor surfaces with continuous wave ultraviolet radiation entitled "Direct Writing in Self-Developing Resists Using Low Power Ultraviolet Light" published in the Journal of Applied Physics on Sep. 15, 1985 found etching depths to be dependent upon fluence (energy density) and independent of the rate of energy deposition (i.e., scanning rate of laser). However, etching rates were very low and the presence of oxygen was required to etch depths as small as 0.15 .mu.m (micrometers).
For example, the organic polymer poly(methyl methacrylate) (PMMA) was exposed to power densities approximately one hundred times less than those known to initiate ablative photodecomposition with excimer lasers. Nevertheless, the material was exposed to the ultraviolet radiation for a much longer period of time so that the energy density (fluence) required to penetrate a 0.15 .mu.m film was 4 kJ/cm.sup.2, which is more than ten thousand times the amount of energy required to etch a similar depth in PMMA by ablative photodecomposition. Even if the spacing between excimer laser pulses is taken into account, the etching rate (measured as depth of material removed per unit of time) by ablative photodecomposition is approximately one hundred times faster than the etching rates produced by the studied continuous wave ultraviolet radiation. (NOTE: The comparative data for etching PMMA with ablative photodecomposition is found in a study I co-authored entitled "Ultraviolet Laser Ablation of Organic Polymers", published in Chemical Reviews, 1989, 89, 1303-1316.)
Although excimer lasers operating in a range that induces ablative photodecomposition have been shown to be far more effective for cutting organic solids than the studied range of cw laser operation, a number of problems remain with ablative photodecomposition processes. For example, impacts of the high energy pulses that are necessary to initiate ablative photodecomposition in organic materials produce audible, often loud reports. Resultant shock waves accompanying the reports can cause damage to organic substrates, especially biological tissue. Also, some of the polymer material that is expelled from an organic substrate by ablative photodecomposition accumulates as debris on the surface of the substrate. A study of this problem was published in the Journal of Applied Physics on Sep. 1, 1986 under the title "The Effect of Debris Formation on the Morphology of Excimer Laser Ablated Polymers".
There are also a number of known disadvantages with using excimer lasers for cutting organic materials. For example, the duty cycle of commercial excimer lasers is less than 0.01 percent of their total operating time. This considerably slows the scanning rate at which the laser can be moved with respect to the organic material. Also, the beam quality of the excimer laser is very poor, and noxious gases such as fluorine or hydrogen chloride are required in a lasing medium.